The formation of myotendinous junctions (MTJs) is a fundamental component of a functional musculoskeletal system. Muscles interact with tendons at their attachment sites, causing a reorganization of the extracellular matrix (ECM) into connective tissue structures that can bear the large forces exerted by muscle contraction. The transcription factors Scleraxis (Scx) and Mohawk (Mkx), as well as the membrane glycoprotein Tenomodulin (Tnmd), specify early tendon progenitors and regulate ECM production, and we recently discovered an ECM protein and Integrin (Itg) ligand called Thrombospondin-4b (Tsp4b) in zebrafish that is regulated by Scx and required for muscle attachment. How tendon progenitors reach sites of muscle attachment and influence ECM assembly and muscle adhesion at the MTJ remains unclear. The current proposal addresses these issues using the advantages of the zebrafish for in vivo imaging and genetic manipulation. The long-term goal of the proposed research is to understand the spatial dynamics of ECM assembly and cell fate specification in tendons and ligaments. Two primary hypotheses guide the research: 1) Mkx and Tnmd interact with Scx to control tenocyte morphogenesis and ECM assembly at MTJs, and 2) Itg and Tgfb signaling in response to mechanical load regulates Scx, Mkx and Tnmd expression during the formation of MTJs. Aim 1 is to visualize tendon progenitors in living wild-type, Mkx and Tnmd mutant embryos. Aim 2 is to study roles for Itg and Tgfb signaling in regulation of Scx, Mkx and Tnmd expression. Aim 3 is to study the roles of Itg and Tgfb signaling in response to mechanical load. Each aim combines novel genetic manipulation, fluorescence dynamics imaging and quantitative methods for physiological stimulation of muscles to get at the mechanisms of ECM assembly at these specialized connective tissue structures.